Mark Harrington, a member of Teamsters Local 25, was arraigned in September on extortion charges related to a TV shoot in Boston.

There was nothing subtle about the labor battle between an electric workers union and a Louisiana utility. Workers fired high-powered rifles at company transformers. They drained oil from company machinery. They blew up a transformer.

And the Supreme Court ruled that their actions didn’t violate federal law because they were part of union activity.

That early 1970s ruling — known as the Enmons decision — helped set parameters for acceptable union activity — a question now at the heart of a federal inquiry into union actions in Philadelphia, Buffalo, and, most recently, Boston.

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In the Boston case, three Teamsters members were sentenced last year to federal prison for threatening to “shut down” events run by businesses that hired nonunion workers.

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In another local pending case, five Teamsters members are accused of disrupting the filming of a television show in an attempt to extort jobs from a production company that hired nonunion workers. A City Hall official has since been indicted in relation to that case, and is also accused of withholding permits to a music festival to force the hiring of union workers.

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In each of the union cases, the union members declared their innocence; their lawyers have cited the Enmons Supreme Court decision as protection for their actions.

The recent prosecutions, and the ongoing federal investigation into union tactics that has reached into City Hall, have led legal observers to wonder: What’s the line between advocacy on behalf of working people and unlawful coercion.

Legal analysts say that the Supreme Court could be asked to revisit the question, saying the 1973 Enmons decision no longer sets the standard in labor prosecutions.

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“It was a case decided in a different time, a different era,” said Wally Zimolong, a Philadelphia-based construction attorney who has monitored the cases there, in Buffalo, and in Boston.

Another point that remains unclear is whether the Enmons decision protects public officials such as Kenneth Brissette, the city’s director of tourism, sports, and entertainment, who was indicted this month for threatening to withhold permits from a music festival that had hired nonunion workers. Brissette would have been acting on behalf of the union, though he wasn’t a member.

Robert Doren, a Buffalo-based labor attorney who followed the prosecutions there, said the indictment suggests Brissette would have been acting in his public capacity, rather than as a union member.

“That goes more to corruption,” he said. “The whole notion of having an exemption . . . has to do with union action, by union officials.”

The question of union rights has particular resonance in East Coast cities where historically powerful unions have struggled to maintain what Zimolong called “their power, their viability, and their relevance,” sometimes leading them to turn to aggressive strategies.

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In Philadelphia just over a year ago, 12 members of the Ironworkers Local 401 were convicted of racketeering and extortion for using violence, including setting fires to construction sites, to intimidate contractors into hiring union members — what prosecutors likened to Mafia-style tactics.

Joseph Dougherty, the then-73-year-old Local 401 business manager, financial secretary, and treasurer, was sentenced to 19 years in prison. About 200 union members rallied on his behalf outside the Philadelphia courthouse before his sentencing in July 2015.

In Buffalo, seven of 12 members of the Operating Engineers Local 17
who were charged with racketeering and extortion in 2008 recently pleaded guilty to vandalizing property and using threats of violence to obtain jobs from nonunion developers.

The threats were related to some of the region’s biggest construction projects, adding millions of dollars in costs to those projects, according to court records. An eighth member was convicted after a trial in 2014. Four were acquitted.

Lawyers and contractors in Philadelphia and Buffalo said the indictments helped curb union threats and harassment that had gone unchecked for years.

“There was this underlying perception that the labor community could get away with aggressive, thug-like tactics, and it didn’t stop until these guys were ultimately indicted and put in jail,” said Brian Sampson, president of the Empire State Chapter of the Associated Builders and Contractors, a construction trade group.

The cases in Boston, Philadelphia, and Buffalo include facts and legal arguments that are most similar to those considered in the Enmons decision, according to legal analysts.

The heart of the issue is the definition of a legitimate labor objective. In the Enmons decision, the Supreme Court found that union members could not be charged with the federal crime of extortion if the underlying act — for example, blowing up a transformer at Gulf State Utilities Co. — was carried out to enhance a legitimate union bargaining position.

But the union member could still be charged in state court with the underlying crime of blowing up a transformer, or destruction of property, but would be protected from prosecution for the federal crime of extortion, which carries a punishment of up to 20 years in prison.

The ruling recognized the long-established rights of unions to protest for better wages.

In its decision, the Supreme Court did not define legitimate labor objectives, but did set parameters for what unions can’t do: Union actions should not lead to payoffs to union members. Also, the ruling did not protect union members who seek to exact wages for what the court described as “imposed, unwanted, superfluous, and fictitious services.”

The ruling was meant to criminalize the act of protesting a company for jobs that do not exist, according to legal analysts.

Legal analysts said that the prosecutions in Boston, Philadelphia, and Buffalo could hinge on the court’s requirement that negotiations involve employers and employees. Unlike the workers in the Enmons case, who were negotiating a new contract, the union members in the recent cases were threatening businesses that did not have a bargaining agreement.

The threat “has to be in the context of a strike, a labor dispute,” said Doren, the lawyer who represented some of the companies targeted by union members in the Buffalo case.

The legal question is likely to be heard by higher courts — two of the defendants in the recent local Teamsters convictions have filed appeals.

They argue that they believed they were carrying out legitimate union activity in seeking a contract with the companies. They also argued that they should be acquitted because the law is unclear.

The question, as it makes its way through the courts, will serve as a backdrop to an ongoing federal investigation into union tactics in Boston.

The Globe reported last month that the investigation has triggered a wave of subpoenas to union leaders, developers, and Boston City Hall staff. Mayor Martin J. Walsh, according to people familiar with the investigation, has also been drawn into the inquiry for his work as a labor leader before taking office in 2014.